Formation mechanism of porous reaction-bonded silicon nitride with interconnected pores in the presence of MgO

In porous reaction bonded silicon nitride, whiskers normally grow in globular clusters as the dominant morphology and deteriorate the pore interconnectivity. However, the ceramic microstructure was significantly transformed with the addition of MgO; specifically, the morphology was modified to a combination of matte and hexagonal grains. Microstructural observation along with thermodynamic studies suggest that MgO interfered with the presence and nitridation of SiO(g). Consequently, rather than being involved in the whiskers' formation, surface silica instead reacted with volatile MgO to form intermediate products. Through these reactions, whisker formation was blocked, and a porous interconnected structure formed which was confirmed by 3D tomography. After heat-treatment at 1700 degrees C,beta-Si3N4 crystallized in a glassy matrix containing magnesium. Resulting samples had an open-pore structure with porosity of 74-84 vol. %, and density of 0.48-0.75 g.cm(-3). Combination of high porosity and pore size of < 40 mu m led to compressive strengths of 1.1-1.6 MPa.